Delegating to a default C++ constructor - c++

Question

Assume a C++ class A with members that can all be copied by the their respective copy constructors. We can rely on the default copy constructor for A to copy its members:
class A {
private:
A(const A&) = default; // We don't really need this line
int a;
B b;
double c;
}
But now let's assume that I want to "extend" (or annotate) the default constructor of A so that in addition to copying its members, it does something else, e.g. writes a line to a log file.
I.e. I'd like to write something like:
class A {
public:
A(const A& a) : A::default(A) {
print("Constructing A\n");
}
private:
// like before
}
Unfortunately that is not correct C++ syntax.
So is there a syntax which allows delegating to a default C++ constructor while explicitly defining the same constructor?

Answer 1

The simplest is to move all members into a base class and write the message in a derived class:
class A_helper {
private:
int a;
B b;
double c;
};
class A : public A_helper {
public:
A() = default;
A(const A& a) : A_helper(a) {
print("Constructing A\n");
}
A(A&& a) : A_helper(std::move(a)) {
print("Constructing A\n");
}
};

Answer 2

You can delegate to the default constructor like you would default-initialize a member variable:
A(const A&) : A()
{
...
}

Answer 3

As often, "We can solve any problem by introducing an extra level of indirection." ( "…except for the problem of too many levels of indirection,"):
struct VerboseA
{
VerboseA() = default;
VerboseA(const VerboseA&) { print("Constructing A\n"); }
VerboseA(VerboseA&&) { print("Constructing A\n"); }
};
And then
class A : VerboseA // EBO (Empty Base Optimisation)
{
private:
A(const A&) = default; // We don't really need this line
int a;
B b;
double c;
};
or in C++20
class A {
private:
A(const A&) = default; // We don't really need this line
[[no_unique_address]]VerboseA verbose; // "EBO" equivalent with attribute
int a;
B b;
double c;
};